Known laser processing machines, e.g. for cutting metallic work pieces, include a laser cutting head on whose end facing the work piece a nozzle is arranged. In the laser cutting head there is a lens that focuses the laser beam. In order to obtain impeccable and uniform cutting results, it would be necessary for the focal point of the bundled laser beam to be at a fixed distance from the work piece surface. Laser processing machines are usually used for cutting out or cutting off contours from sheet metal parts. Processing such work pieces takes place via a laser beam, emanating from an opening of the nozzle, directed onto the work piece to be processed, and guided along a predetermined cutting contour by means of a main drive of the machine.
In terms of the quality of work, that is presently achievable, precise adjustments of the focus position and of the point of impact of the laser beam on the work piece play a decisive role. Thus, to obtain an optimal cutting process with laser cutting machines, the laser beam should be centrically guided through the nozzle of a cutting head, a requirement which is not easy to meet in a practical application. Because lateral displacement of the lenses occurs in the cutting head, caused by, for example, changing the lens or relative sliding of the lenses, one of the lenses may cause a relative displacement of the optical axis. As a result, regular checks and centering and focusing steps are necessary.
Conventionally, the nozzle of laser cutting machines is adjusted in such a manner that the nozzle body is centered in X-Y directions relative to the nozzle head and is then fixed. Such a nozzle centering is carried out by means of adjusting screws or other additional adjusting devices in the X axis or Y axis on the cutting head or on lens slide-in units. Adjusting the focus position (focusing) in the Z axis is usually carried out manually by means of an adjusting screw.
In the present description the term “adjustment” refers on the one hand to nozzle centering in the plane of the nozzle aperture (i.e., positioning of the nozzle in the directions of the X-Y axes), and on the other hand to laser focusing, i.e. adjustment of the focus position (in the direction of the Z axis).
EP-1,561,538 (A1) describes a device for adjusting a laser beam in a laser processing machine. The device includes a work piece receiving unit for receiving and bearing the work piece to be processed, and a laser cutting head. To adjust the laser beam, an alignment unit is provided. The alignment unit is arranged in the direction of propagation of the laser beam in a test station, wherein the laser cutting head is positioned. The alignment unit includes a nozzle fixing element, a projection element for acquiring a projection of the laser beam in an image plane, and an evaluation or comparison unit. In order to facilitate adjustment of the laser beam, in addition an image pattern in the form of a target is overlaid on the monitor of the laser beam, which target shows the desired position of the laser beam as a central point. In this arrangement, position corrections of the laser beam also take place manually by way of adjustment screws.
The above-mentioned manual adjustment of the laser beam by means of adjustment screws and additional adjustment devices installed between the nozzle and the cutting head is too cumbersome, requires considerable time and overall is not suitable to provide adequate cutting quality in accordance with modern requirements.
JP-H10-249,566(A) discloses a laser beam machine for machining of work pieces by a laser beam emitted from a laser generator by means of a condensing lens and by emitting the laser beam to a work piece through a nozzle. The luminance of the laser beam axial center is detected by a sensor having a photo-electric conversion element for making a comparison between the luminance data thus detected and a set luminance at the time when the axial center of the laser beam is preliminarily in the center of the nozzle, so that a deviation is detected between the center of the nozzle and the axial center of the irradiating light axis. On the basis of the deviation data, the nozzle is adjusted by means of an actuator to align the center of the nozzle with the axial center of the irradiating light axis.
But, it is to be noted that by means of this luminance detecting of the laser beam center a really precise and reproducible processing of work pieces of higher quality requirements cannot be carried out.